as the guy said, login to your ipod using your cuteftp and look for /private/var/root/Library/SummerBoard/Themes.

you need to make sure that when you put the folder in you dont just drag the whole zip folder. you need to make sure you have another folder with the name of your theme so /private/var/root/Library/SummerBoard/Themes/MYTHEME/

put the dock and wallpaper pngs straight into MYTHEME but make another folder called icons containing, naturally enough, all the icons

if you look at any of the themes you already have you will understand the structure

Before following any of these steps, be sure that your iPod is connected to your wireless home network. Do not let your iPod go into standby while following these steps. It can cause some minor issues. (Just tap your screen every 30 seconds or so...)

Step by Step:
1. For this guide, we are going to be using WinSCP.
2. Open up WinSCP and a window will ask you for a: Host name
Port Number
Username
Password
3. To find your Hostname, tap 'Settings' followed by 'Wi-Fi'.
4. Next, tap on the arrow to the right of the network you are connected to. You will find a field saying 'IP Address' with a number to the right. This is the number you will be typing into the 'Host Name' field on WinSCP.
5. The next field says 'Port Number' and should be set to 22.
6. The default username is root and the default password is alpine.
7. Next, click 'Login'.
8. Now, a window will pop up showing the directories of your iPod. You can now successfully browse through the folders and subfolders of your iPod on your computer. To add a file from your computer onto your iPod, you can copy and paste files from your computer into the appropriate folders on your iPod.

How to Set a Theme

In WinSCP, go to the following directory:/private/var/root/Library/SummerBoard/Themes/

In this directory, create a new folder called:ThemeFolder

Drag the following files into this new folder you created:Wallpaper.png
Dock.png
StatusBar.png

There may be no StatusBar.png depending on if the creator made one or not.

Now, if the theme has icons, create a new folder inside ThemeFolder called Icons. Place all of the icons inside this folder.

Once done, you can always change ThemeFolder to whatever you want, specifically the name of the Theme you are using.